Continental Drift and Plate Tectonics sample essay
Introduction: The Beginning of the “Continental Drift Theory” In the middle of the eighteenth century, James Hutton proposed a theory, uniformitarianism; “the present is the key to the past”. It held that processes such as geologic forces- gradual and catastrophic-occurring in the present were the same that operated in the past. (Matt Rosenberg, 2004) This theory coincides with the theory of Continental Drift that was first proposed by Abraham Ortelius in December 1596, who suggested that North, South America, Africa and Eurasia were once connected but had been torn apart by earthquakes and floods.
He also discovered that the coasts of the eastern part of South America and the western coasts of Africa fit together like a jigsaw puzzle and this fit becomes especially prominent as the edges of the continental shelves have similar shapes and thus, appear to be once fitted together. (Figure 1.1 and Figure 1.2) The similarity of southern continents’ geological formations had led Roberto Mantovani to speculate that all continents had once been a supercontinent and was smaller in its volume than it is now. Through volcanic activity, fissures are created in the crust causing this continent to break apart. However, this theory, known as the Expanding Earth Theory has since been proven incorrect.
The Theory of Continental Drift
In 1912, The Theory of Continental Drift was intensively developed by Alfred Wegener, who claimed that the world was made up of a single gigantic supercontinent named Pangea since the Permian period, 250 million years ago. It began forming at the beginning of the Carboniferous period, 365 million years ago, when Gondwana collided into Laurussia producing the Appalachian mountain belt in eastern North America and closing in Paleo-Tethys Ocean and modern landmass became exposed to air. Alexander Du Toit then suggested that 145-200 million years ago, in the middle Jurassic Period, Pangea started breaking up into two smaller supercontinents, Laurasia in the northern hemisphere and Gondwana in the southern hemisphere, with Tethys Sea and North Atlantic Ocean separating the two supercontinents.
The late Jurassic era began the formation of the Rocky Mountains and Sierra Nevada mountains. In the Cretaceous Period, 65 million years ago, the two supercontinents then began fragmenting into the present seven continents. (USGS, 2012) The Tethys Sea that lay between the two landmasses was subducted beneath Eurasia, forming the lower Atlantic Ocean. Eventually, it disappeared. (Nelson Thomas, 2007) (Figure 2) Wegener proposed that continents were moving at about one yard per century and supported this theory with several points of evidence.
Evidence supporting the Theory of Continental Drift (Alfred Wegener and Du Toit) Alfred Wegener matched up coastlines, and he realized that by fitting the continental shelves together, cratons formed a contiguous pattern across the boundary of South America and Africa. (Lois Van Wagner, 2013) He realized that mountain ranges that ended at one coastline seemed to begin again on another such as ancient mountains in South Africa that align with the mountains in near Buenos Aires in Argentina. (Sant, Joseph, 2012) He discovered earthworms of the family Megascolecina, who are unlikely to be long-distance migrators, were found in soils of all the Gondwanaland continents. (kangarooistan, 2009) This identical species could not have arisen on different continents without some variations. (WiseGeek, 2010) Fossil remains of a prehistoric reptile known as the Mesosaurus had been uncovered on both sides of the South Atlantic coasts, yet the creature was unable to swim across the Atlantic Ocean. (Lois Van Wagner, 2013) Fossils of the land reptile, Lystrosaurus were discovered in South America, Africa and Antarctica. (Sant, Joseph, 2012).
He also discovered the fossil plant Glossopteris was distributed throughout India, South America, Southern Africa, Australia and Antarctica. (USGS, 2012)(Figure 3) Alexander Du Toit traveled to Brazil and Argentina where he found similarities in the fossils and rock strata to those found in South Africa such as the fossilized remains of Mesosaurus in fresh water deposits, dune deposits capped by basalt flows, tillite and coal beds. Similar layers of rock were formed in Antarctica, Australia, South America, Africa and India. (Figure 4) Widespread distribution of Permo-Carboniferous glacial sediments in South America, Africa, Madagascar, Arabia, India, Antarctica and Australia and striations that indicated glacial flow away from the equator and towards the poles were discovered and supported the theory of Continental Drift which proposed that southern continents were once located over the South Pole region and covered by ice sheets. (Lois Van Wagner, 2013) (Figure 4)
He also discovered a base layer of shale scratched by glaciers and covered by layers of tillite in South Africa, a continent of a tropical equatorial climate. Tillites and varves dating back to 2 billion years ago, were found in Canada and India, indicating glaciation on a worldwide scale. Such tillites were found on all major continents except Antarctica, which has been the most extensive glacial continent in earth’s history. (kangarooistan, 2009) Additionally, fossils of tropical plants in the form of coal deposits were found in Antarctica which implies that Antarctica had to be closer to the Equator. (USGS, 2012) This study of changes in climate taken on the scale of the entire history of Earth is known as paleoclimatology. Sediments of rifting have proved the drifting apart of Pangea.
The rifting that formed the South Atlantic Ocean began late in the Mesozoic Period when Africa and South America began to pull apart. Water from the south then flowed in over time, thus forming the evaporites now found along the coastlines there. (Lois Van Wagner, 2013)(Figure 5) However, Wegener believed that only the continents were moving and they plowed through the rocks of the ocean basins. (Colliers Encyclopedia, 1996) Harold Jeffreys then argued that it is impossible for continents to break through solid rock without breaking apart. (USGS, 2012) Wegener also claimed that the centrifugal force of the spinning planet had forced the continents sideways, parallel to the equator; tidal pull from the sun and moon had caused lateral movement. (Sant, Joseph, 2012) His orders of magnitude were too weak. Thus, his theory was dismissed. (Lois Van Wagner, 2013) Further development and support of the “Continental Drift Theory” in the 1960s After World War 2, the U.S. Office of Naval Research intensified efforts in ocean-floor mapping, leading to the discovery of the Mid-Atlantic Ridge to be part of a continous system of mid-oceanic ridges on all ocean floors, prompting Harry H. Hess to suggest the theory of sea-floor spreading.
The oldest fossils found in ocean sediments were only 180 million years old and little sediment were accumulated on the ocean floor. Thus, he suggested that seafloors were no more than a few hundred million years old, significantly younger than continental land due to hot magma rising from volcanically active mid-oceanic ridges, spreading sideways, cooling on the seafloor’s surface due to cooler temperatures of the sea, solidifying to create new seafloor, thereby pushing the tectonic plates apart. (Edmond A. Mathez, 2000) The realization that the shape of the Mid-Atlantic Ridge and the Atlantic Coast are strikingly similar substantiated the claim that the continents had been joined together at the Mid-Atlantic Ridge. (J. Tuzo Wilson, 1996) (Figure 6)
The cause of the continental drift that Wegener was unable to explain had been further researched on by Arthur Holmes who claimed that the movement of continents was the result of convection currents driven by the thermal convection in the heat of the interior of the Earth, namely the mantle. The heat source of the mantle comes from radioactivity decay in the core. (Figure 7) At constructive plate boundaries, molten basalt flows out on either side of the ridge and cools with the iron particles in the basalt aligning with the earth’s magnetic field which reverses direction every few hundred thousand years. (Lois Van Wagner, 2013) Due to magma cooling, the polarity of rocks will be recorded at the time it was formed. (Figure 8.1)
In 1950, researchers of paleomagnetism discovered that there were alternating regions of normal and reversed magnetic directions symmetrically disposed on both sides of the Mid-Atlantic Ridge –magnetic stripping. (J. Tuzo Wilson, 1996) Harry H. Hess’ theory was thus proven by the magnetic anomalies in the oceanic crust. (Nelson Thomas, 2007) (Figure 8.2) It was also discovered that the youngest rocks were closest to the mid-oceanic Ridge and the oldest rocks were near the coasts of the continents. When scientists began collecting magnetic data for North America and Europe, they discovered the north pole seemed to be moving about over time. (ALLA, 2009) However, when data from other continents was collected for the same time frames, it showed different polar locations, thus supporting that continents were moving about.
The Theory of Plate Tectonics
The theory of plate tectonics held that the Earth’s lithosphere, the Earth’s crust and the uppermost mantle, is broken into seven macro-plates and about twelve smaller ones, averaging 50 miles in width. (U.S. Dept. of the Interior, Geological Survey, 2007) Any plate may consist of both oceanic crust and continental crust. (Colliers Encyclopedia, 1996) (Figure 9) It suggests that the ocean floor began to spread at constructive plate boundaries, and continents, existing on “plates”, moved due to convection currents in the mantle and constant sea-floor spreading. (The Columbia Electronic Encyclopedia, 2011). They drag and move plates above them due to rising magma spreading out beneath the earth’s crust. As two oceanic plates move apart, magma from the underlying asthenosphere mantle wells up from oceanic ridges and becomes rigid enough to join the lithosphere of the plates on either side of the plate boundary, creating new seafloor and eventually, an ocean is opened up. (J. Tuzo Wilson, 1996) (Figure 10) Examples are the Atlantic Ocean formed between South America and Africa.
New rock is created by volcanism at mid-oceanic ridges and returned to the Earth’s mantle at oceanic trenches where the denser plate is subducted under the other, forcing the earth’s crust back into the mantle. (J. Tuzo Wilson, 1996) This process is known as the ridge push and slab-pull. (Figure 11) Different plate tectonics movement and subsequent tectonic activities Transform plate movement causing earthquakes: Seismic waves disrupting the continents in the form of earthquakes are due to the great amount of stress and energy built up by the friction of the moving plates, especially during transform plate movement, where plates slide past each other in a grinding, shearing manner and form tear faults (Columbia Electronic Encyclopedia, 2011). (Figure 12.1) There is gradual bending of rocks before the ductile limit of rocks is exceeded, causing the plates to lock and the fault to break, leading to sudden release of stored energy, causing earthquakes. (Nelson Thomas, 2007)
An example is the strike-slip fault, San Andreas Fault in California. (Figure 12.2) (WiseGeek, 2010) Oceanic and Oceanic convergent plate movement: Other evidence of plate tectonics movement are most of the world’s active volcanoes located along or near the boundaries between shifting plates known as plate-boundary volcanoes. (J. Tuzo Wilson, 1996) When two oceanic plates collided, the denser plate will subduct under the other, forming a deep oceanic trench and form magma through hydration or decompression melting. The magma being less dense than the surrounding mantle, rises and escapes to the sea-floor through cracks in the earth’s crust, forming submarine volcanoes that rise above water to form a chain of volcanic islands known as island arcs, such as the Japan Islands. (Figure 13) Examples would be the Pacific Plate subducting underneath the North American Plate creating the Kuril Trench and the Japan Trench that can be found along the Pacific Ring of Fire.
Many volcanoes such as Mount St. Helens, Mount Fuji in Japan and Mount Pinatubo in the Phillipines are located along the perimeter of the Pacific Ocean Basin where boundaries of several plates such as the Nazca and the Cocos Plate are found, forming the Ring of Fire. (Fraser Cain, 2009) (Figure 14) Volcanoes formed not due to tectonic activities: 5 per cent of the world’s volcanoes are formed at isolated “hot spots” and many intra-plate volcanoes form roughly linear chains along the middle of oceanic plates. (The Columbia Electronic Encyclopedia, 2011)Examples are the Yellowstone National park and Hawaiian Islands, an intra-plate volcanic chain developed by the Pacific plate passing over a deep, stationary “hot spot”, located 60 km beneath the present-day position of the Island of Hawaii. Heat from this hotspot produced a constant source of basaltic magma by partly melting the overriding Pacific Plate.
This magma rises through the mantle to erupt onto the seafloor, forming an active seamount. Over time, countless eruptions caused the seamount to grow until it finally emerges above sea level to form island volcanoes. The continuing plate movement eventually carries the island volcano away from the hotspot, cutting it off from the “hot spot” and creating another island volcano. This cycle is repeated, forming the Hawaiian Islands. (U.S. Dept. of the Interior, Geological Survey, 2007) (Figure 15) Continental and Continental convergent plate movement: Continental fold mountain ranges are evidence of two continental plates that are thick and buoyant thus, preventing both plates from subducting. Instead, the two plates collide into each other forming fold mountain ranges in a process known as orogenesis.
An example is the high elevation of the Tibetan plateau, fringed to the south by the Himalayas as the edges of the Indian and Eurasia plate buckle, uplift, fold and deform. Mt. Everest is the highest summit on Earth, yet Yellowband limestone that was originally part of the shallow seals of the Tethys Ocean was found on Mount Everest at a height of 8462m. (Figure 16) Oceanic and Continental convergent plate movement: Mountains are formed when oceanic crust is subducted under a continental crust, resulting in melting of rock, thus volcanic activity and causing the continental crust to deform, rise and buckle upwards under compressional forces.
Examples are the Andes Mountain, the Chile-Peru Trench and the uplift of the Rockies and Appalachians in the past. (The Columbia Electronic Encyclopedia, 2007) The Table Mountains was formed approximately 250 million years ago, due to the Pacific plate subducting under the North American plate, (Mary Ann Resendes, 2012) thus creating the Sierra Nevada foothills, subsequently creating the Cape of Good Hope as the ocean erodes the soft sandstone of Table Mountains on the coast. (National Geographic, 1996)
Other tectonic activities such as the Wadati-Benioff zones, that are earthquake zones parallel to oceanic trenches are also formed at such subduction zones and inclined from 40 to 60 degrees from the horizontal, extending several hundred kilometres into the mantle. (Figure 17) Continental and Continental divergent plate movement: When two continental crusts are pulled apart due to tensional forces, the area sinks and forms a rift valley and sea such as the East African Rift Valley and the Red Sea that runs from the Jordan Valley and into East Africa, already dotted with volcanoes such as Hermon. This is due to the area being stretched, causing the crustal material to thin, weaken and sink due to lowered density. (Figure 18)
Also, isostasy takes place wherever a large amount of weight such as the fold mountain ranges created from plate tectonics movements is formed or glaciers, pushes down the Earth’s crust and creates a small dent. Isostasy also takes place at divergent plate boundaries when a large amount of weight is removed from an area, causing that portion of the Earth’s crust to rise. Therefore, equilibrium in the earth’s crust is achieved such that forces elevating landmasses balances those tending to depress landmasses. (Learning Network, 1998) (Figure 19)
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